[$] Avoiding page reference-count overflows

The 5.1-rc5 announcement mentioned"changes all over" and highlighted a number of the areas thathad been touched. One thing that was not mentioned there was theaddition of four patches fixing a security-related issue in the corememory-management subsystem. The vulnerability is sufficiently difficultto exploit that almost nobody should feel the need to rush out a kernelupdate, but it is still interesting to look at as a demonstration of howthings can go wrong.